Investigation On Microstructure And Properties Of Marine Metal Materials Remanufactured By The Underwater Laser Direct Metal Deposition Technique

The 21st century is the century of the ocean.The marine engineering materials,including titanium alloys and high-strength low-alloy steels,have been widely used in manned submersibles,advanced ships and offshore drilling platforms.Those materials have very excellent application prospects.Marine engineering materials are easily damaged during their service in the seawater environment,which greatly threatens the service safety of marine engineering equipment.The development of new technologies for underwater repair of marine engineering equipment can be an important support for the development of marine resources and deep-sea exploration.The traditional underwater arc welding technique has the characteristics of large heat affected zone and poor repair quality,which is difficult to meet the modern restoration requirements.As a new type of rapid prototyping technology,laser additive remanufacturing possesses the characteristics of high energy density,controllable heat input and small heat-affected zone.Small deformation and residual stress are formed on the damaged components repaired by additive manufacturing.It is meaningful to introduce in-air direct metal deposition(in-air DMD)technique into the water environment to create a new technique,namely underwater direct metal deposition(UDMD).By combining the laser cladding head with the underwater motion robot,it is expected to realize the automation repair of damaged marine engineering components via UDMD.It is believed that UDMD can improve the repair quality and prolong the service life of marine equipment.In this work,the on-site repair of titanium alloy Ti-6Al-4V and high-strength low-alloy steel HSLA-100 was performed by UDMD.The as-repaired appearances,micro structural formation/evolution,mechanical properties and corrosion resistance of the samples repaired in the shallow water depths and the 30 m water depth were studied in detail.The differences in metallurgical processes,microstructure and mechanical properties of the UDMD and in-air DED processes were compared.The specific research contents and results of this work are listed as follows.(1)The Ti-6Al-4V bulk samples were successfully fabricated by UDMD in a water tank with a water depth of 60 mm.Correlation mechanisms between the UDMD process,defects formation,micro structural formation/evolution and tensile properties of bulk Ti-6Al-4V were clarified.The defects,phase composition,micro structure and mechanical properties of the bulk samples were systematically analyzed.The results show that fully dense bulk samples can be fabricated using the optimized process parameters(laser power 1600 W,scanning speed 800 mm/min and cross-hatching strategy).The micro structure of the Ti-6Al-4V repaired by UDMD was composed of columnar prior-β grains and acicular α’ martensite.Along the build direction of the sample,the grain size of α’ martensite decreased,and the density of lattice defects in the grain increased.For the specimens extracted from the bottom region of the block,they exhibited the best comprehensive mechanical properties.The qualities(both micro structure and mechanical properties)of the Ti-6Al-4V repaired by UDMD were equal or even better than that fabricated by in-air DMD or selective laser melting technique.(2)The prepared trapezoidal grooves on the Ti-6Al-4V plates were successfully remanufactured by UDMD at a water depth of 60 mm.The correlation mechanisms between the UDMD remanufacturing process,temperature history,elemental diffusion,micro structural formation/evolution,tensile properties and impact properties were elucidated.The relationship between the UDMD remanufacturing process,microstructure and mechanical properties was systematically studied via experimental and numerical methods.The results show that the micro structural formation/evolution and element diffusion behaviors in Ti-6Al-4V strongly depended on the manufacturing environment(underwater/in air).The V content in the β films of the UDMD sample was lower than that of the in-air DMD sample.The extremely high cooling rate and low thermal accumulation restricted the diffusion of atoms,resulting in the formation of thin β films.The microstructure of the samples repaired by UDMD was mainly composed of fine acicular martensite α’ and thinner β film layers.The yield strength and ultimate tensile strength of the samples repaired by UDMD were lower than those of the samples repaired by in-air DMD.The low absorbed energy of the samples(14 J)repaired by UDMD was ascribed to the rapid propagation of cracks in the columnar prior-β grain boundaries and the brittle acicular martensite α’ within the grain boundaries.(3)The correlation mechanisms between the micro structure,fatigue properties and corrosion resistance properties of the Ti-6Al-4V remanufactured by UDMD were clarified.The fatigue properties of the Ti-6Al-4V repaired by UDMD were tested using an in-situ SEM fatigue testing machine.The corrosion resistance of the Ti-6Al-4V repaired by UDMD was also investigated.The fatigue lifetime of the Ti-6Al-4V repaired by UDMD was shorter than those of the Ti-6Al-4V repaired by in-air DMD and substrate.The fine acicular α’ martensite in the UDMD sample caused stress concentration under the cyclic loading and contributed to the fast nucleation of micro crack.The Ti-6Al-4V repaired by UDMD exhibited a transgranular fracture mode during the short fatigue crack propagation process.The fine α’ laths in the Ti-6Al-4V repaired by UDMD cannot prevent crack propagation,leading to a fast fatigue crack growth rate.In addition,the corrosion resistance of the Ti-6Al-4V repaired by UDMD was better than that of the Ti-6Al-4V repaired by in-air DMD.The corrosion resistance of the Ti-6Al-4V repaired by UDMD was mainly determined by three factors:the grain size of the microstructure,the distribution of alloy elements and the surface structure state.(4)A high-performance and defect-free repaired zone was achieved on the HSLA-100 plates by UDMD at water depths ranging from 20 mm to 150 mm.The correlation mechanisms between UDMD remanufacturing parameters,water depth,repair quality,micro structural formation/evolution,tensile properties and impact properties were investigated.The results show that compared with in-air DED,the underwater environment facilitated the fast cooling of the melt pool.The peak temperature and holding time of the melt pool in the sample repaired by UDMD were lower than those in the sample repaired by in-air DMD.The thermal accumulation in the sample repaired by UDMD was much lower than that in the sample repaired by in-air DMD,which resulted from the intense heat dissipation effect by water.The rapid cooling associated with additive manufacturing and the water quenching effect contributed to the formation of fine martensite laths for the sample repaired by UDMD.No obvious transition or dynamic recrystallization occurred in the fresh martensite owing to the low temperature of deposits and substrate cooled by water.The tensile strength and impact toughness of the samples by UDMD were comparable to those repaired by in-air DMD.The repair quality under different water depths was the same as the onshore repair quality.(5)High-performance remanufacturing of HSLA-100 steel plates was successfully achieved by UDMD at an ambient pressure of 0.3 MPa.The correlation mechanisms of UDMD processing parameters,water depth,repair quality,microstructural formation/evolution,tensile properties and impact properties were revealed.The results show that high deposition efficiency can be obtained by adjusting the underwater laser processing parameters.Due to the influence of the fast cooling rates and rapid solidification of the melt pool,lath martensite with a high dislocation density was formed.A large number of inclusions were produced in the samples repaired by UDMD due to the disturbance of the gas curtain gas and the fast cooling rates.The impact absorbed energies of the samples repaired by UDMD were lower than those of the samples repaired by in-air DMD due to the fine fresh martensite,high dislocation density and a great deal of inclusions in the samples repaired by UDMD.In this work,the relationship between the underwater environment and laser-material interaction process was clarified.The results of this work can provide some experimental basis and theoretical basis for promoting efficient underwater repair of marine engineering equipment.This work is also expected to serve the strategic objective of maritime power in China.